Systems and methods having an optical magnetometer array with beam splitters

ABSTRACT

An array of optically pumped magnetometers includes an array of vapor cells; and an array of beam splitters. The array of beam splitters is arranged into columns, including a first column, and rows. Each row and each column includes at least two of the beam splitters. The array of beam splitters is configured to receive light into the first column of the array and to distribute that light from the first column into each of the rows and to distribute the light from each of the rows into a plurality of individual light beams directed toward the vapor cells.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/984,752, filed Aug. 4, 2020, which claims the benefit of U.S.Provisional Patent Application Ser. Nos. 62/883,406, filed Aug. 6, 2019,and 62/896,929, filed Sep. 6, 2019, all of which are incorporated hereinby reference in their entireties.

FIELD

The present disclosure is directed to the area of magnetic fieldmeasurement systems including systems for magnetoencephalography (MEG).The present disclosure is also directed to methods and systems having anarray of optically pumped magnetometers (OPM) and an array of beamsplitters for applications such as high spatial resolution MEG.

BACKGROUND

In the nervous system, neurons propagate signals via action potentials.These are brief electric currents which flow down the length of a neuroncausing chemical transmitters to be released at a synapse. Thetime-varying electrical currents within an ensemble of neurons generatesa magnetic field. Magnetoencephalography (MEG), the measurement ofmagnetic fields generated by the brain, is one method for observingthese neural signals.

Existing technology for measuring MEG typically utilizes superconductingquantum interference devices (SQUIDs) or collections of discreteoptically pumped magnetometers (OPMs). SQUIDs require cryogenic cooling,which is bulky, expensive, requires a lot of maintenance. Theserequirements preclude their application to mobile or wearable devices.

An alternative to an array of SQUIDs is an array of OPMs. For MEG andother applications, the array of OPMS may have a large number of OPMsensors that are tightly packed. Such dense arrays can produce a highresolution spatial mapping of the magnetic field, and at a very highsensitivity level. Such OPMs sensors can be used for a wide range ofapplications, including sensing magnetic field generated by neuralactivities, similar to MEG systems.

BRIEF SUMMARY

One embodiment is an array of optically pumped magnetometers thatincludes an array of vapor cells; and an array of beam splitters. Thearray of beam splitters is arranged into columns, including a firstcolumn, and rows. Each row and each column includes at least two of thebeam splitters. The array of beam splitters is configured to receivelight into the first column of the array and to distribute that lightfrom the first column into each of the rows and to distribute the lightfrom each of the rows into a plurality of individual light beamsdirected toward the vapor cells.

In at least some embodiments, the array of vapor cells is an N×M arrayand the array of beam splitters is an (N+1)×M array, wherein N and M areintegers greater than one. In at least some embodiments, the array ofbeam splitters is configured to generate N×M beams of light. In at leastsome embodiments, the N×M beams of light have intensities that differ byno more than 5% from each other.

In at least some embodiments, the first column has M of the beamsplitters and the m-th one of the beam splitters in the first column hasa reflectivity of 1/(M−m+1), wherein M is an integer greater than oneand m is an integer ranging from 1 to M. In at least some embodiments,at least one row has a one of the beam splitters from the first columnfollowed by N of the beam splitters after, wherein the n-th one of the Nbeam splitters has a reflectivity of 1/(N−n+1), wherein N is an integergreater than one and n is an integer ranging from 1 to N.

In at least some embodiments, the array of optically pumpedmagnetometers further includes a quarter waveplate disposed between thearray of beam splitters and the array of vapor cells. In at least someembodiments, the array of optically pumped magnetometers furtherincludes a light source configured and arranged to direct light into thefirst column of the array of beam splitters. In at least someembodiments, the array of optically pumped magnetometers furtherincludes a reference detector configured to receive light that haspassed through the beam splitters of the first column.

In at least some embodiments, the beam splitters are polarizing beamsplitters. In at least some embodiments, the array of optically pumpedmagnetometers further includes waveplates disposed between adjacent onesof the polarizing beam splitters to rotate a polarization of a lightbeam exiting one of the polarizing beam splitters prior to enteringanother one of the polarizing beam splitters.

In at least some embodiments, the beam splitters are bonded togetherinto a single block using optical adhesive.

Another embodiment is a magnetic field measurement system that includesan array of vapor cells; an array of light detectors configured toreceive light passing through the vapor cells; and an array of beamsplitters. The array of beam splitters is arranged into columns,including a first column, and rows. Each row and each column includes atleast two of the beam splitters. The array of beam splitters isconfigured to receive light into the first column of the array and todistribute that light from the first column into each of the pluralityof rows and to distribute the light from each of the rows into aplurality of individual light beams directed toward the vapor cells.

In at least some embodiments, the magnetic field measurement systemfurther includes at least one magnetic field generator disposed aroundat least one of the vapor cells to generate a magnetic field in thevapor cell. In at least some embodiments, the magnetic field measurementsystem further includes a light source configured and arranged to directlight into the first column of the array of beam splitters. In at leastsome embodiments, the magnetic field measurement system further includesa computing device coupled to the array of light detectors and the lightsource.

In at least some embodiments, the magnetic field measurement systemfurther includes a wearable article within which the array of vaporcells, array of light detectors and array of beam splitters aredisposed. In at least some embodiments, the magnetic field measurementsystem further includes a light source disposed in the wearable articleand configured and arranged to direct light into the first column of thearray of beam splitters.

In at least some embodiments, the first column has M of the beamsplitters and the m-th one of the beam splitters in the first column hasa reflectivity of 1/(M−m+1), wherein M is an integer greater than oneand m is an integer ranging from 1 to M. In at least some embodiments,at least one row has a one of the beam splitters from the first columnfollowed by N of the beam splitters after, wherein the n-th one of the Nbeam splitters has a reflectivity of 1/(N−n+1), wherein N is an integergreater than one and n is an integer ranging from 1 to N.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following drawings. In the drawings,like reference numerals refer to like parts throughout the variousfigures unless otherwise specified.

For a better understanding of the present invention, reference will bemade to the following Detailed Description, which is to be read inassociation with the accompanying drawings, wherein:

FIG. 1A is a schematic block diagram of one embodiment of a magneticfield measurement system, according to the invention;

FIG. 1B is a schematic block diagram of one embodiment of amagnetometer, according to the invention;

FIG. 2 shows a magnetic spectrum with lines indicating dynamic ranges ofmagnetometers operating in different modes;

FIG. 3A is a perspective view of one embodiment of an array of beamsplitters for use with an array of magnetometers, according to theinvention;

FIG. 3B is a top view of the array of beam splitters of FIG. 3A,according to the invention;

FIG. 3C is a front view of one row of the array of beam splitters ofFIG. 3A, according to the invention;

FIG. 4 is a perspective view of one embodiment of an array ofmagnetometers including the array of beam splitters of FIG. 3A,according to the invention;

FIG. 5 is a top view of another embodiment of an array of beam splittersfor use with an array of magnetometers, according to the invention;

FIG. 6 is a top view of a third embodiment of an array of beam splittersfor use with an array of magnetometers, according to the invention;

FIG. 7A is a perspective view of a prism for use in an array of beamsplitters, according to the invention;

FIG. 7B is a perspective view of several of the prisms coated for use inan array of beam splitters, according to the invention;

FIG. 7C is a perspective view of an arrangement of different prisms foruse in an array of beam splitters, according to the invention; and

FIG. 7D is a perspective view of an arrangement of different prismsbonded for use in an array of beam splitters, according to theinvention.

DETAILED DESCRIPTION

The present disclosure is directed to the area of magnetic fieldmeasurement systems including systems for magnetoencephalography (MEG).The present disclosure is also directed to methods and systems having anarray of optically pumped magnetometers (OPM) and an array of beamsplitters for applications such as high spatial resolution MEG.

Herein the terms “ambient background magnetic field” and “backgroundmagnetic field” are interchangeable and used to identify the magneticfield or fields associated with sources other than the magnetic fieldmeasurement system and the magnetic field sources of interest, such asbiological source(s) (for example, neural signals from a user's brain)or non-biological source(s) of interest. The terms can include, forexample, the Earth's magnetic field, as well as magnetic fields frommagnets, electromagnets, electrical devices, and other signal or fieldgenerators in the environment, except for the magnetic fieldgenerator(s) that are part of the magnetic field measurement system.

The terms “gas cell”, “vapor cell”, and “vapor gas cell” are usedinterchangeably herein. Below, a gas cell containing alkali metal vaporis described, but it will be recognized that other gas cells can containdifferent gases or vapors for operation.

The methods and systems are described herein using optically pumpedmagnetometers (OPMs). While there are many types of OPMs, in generalmagnetometers operate in two modalities: vector mode and scalar mode. Invector mode, the OPM can measure one, two, or all three vectorcomponents of the magnetic field; while in scalar mode the OPM canmeasure the total magnitude of the magnetic field.

Vector mode magnetometers measure a specific component of the magneticfield, such as the radial and tangential components of magnetic fieldswith respect the scalp of the human head. Vector mode OPMs often operateat zero-field and may utilize a spin exchange relaxation free (SERF)mode to reach femto-Tesla sensitivities. A SERF mode OPM is one exampleof a vector mode OPM, but other vector mode OPMs can be used at highermagnetic fields. These SERF mode magnetometers can have high sensitivitybut may not function in the presence of magnetic fields higher than thelinewidth of the magnetic resonance of the atoms of about 10 nT, whichis much smaller than the magnetic field strength generated by the Earth.

Magnetometers operating in the scalar mode can measure the totalmagnitude of the magnetic field. (Magnetometers in the vector mode canalso be used for magnitude measurements.) Scalar mode OPMs often havelower sensitivity than SERF mode OPMs and are capable of operating inhigher magnetic field environments.

The magnetic field measurement systems, such as a biological signaldetection system, described herein can be used to measure or observeelectromagnetic signals generated by one or more magnetic field sources(for example, neural signals or other biological sources) of interest.The system can measure biologically generated magnetic fields and, atleast in some embodiments, can measure biologically generated magneticfields in an unshielded or partially shielded environment. Aspects of amagnetic field measurement system will be exemplified below usingmagnetic signals from the brain of a user; however, biological signalsfrom other areas of the body, as well as non-biological signals, can bemeasured using the system. In at least some embodiments, the system canbe a wearable MEG system that can be portable and used outside amagnetically shielded room.

A magnetic field measurement system, such as a biological signaldetection system, can utilize one or more magnetic field sensors.Magnetometers will be used herein as an example of magnetic fieldsensors, but other magnetic field sensors may also be used in additionto, or as an alternative to, the magnetometers. FIG. 1A is a blockdiagram of components of one embodiment of a magnetic field measurementsystem 140 (such as a biological signal detection system.) The system140 can include a computing device 150 or any other similar device thatincludes a processor 152, a memory 154, a display 156, an input device158, one or more magnetometers 160 (for example, an array ofmagnetometers) which can be OPMs, one or more magnetic field generators162, and, optionally, one or more other sensors 164 (e.g., non-magneticfield sensors). The system 140 and its use and operation will bedescribed herein with respect to the measurement of neural signalsarising from one or more magnetic field sources of interest in the brainof a user as an example. It will be understood, however, that the systemcan be adapted and used to measure signals from other magnetic fieldsources of interest including, but not limited to, other neural signals,other biological signals, as well as non-biological signals.

The computing device 150 can be a computer, tablet, mobile device, fieldprogrammable gate array (FPGA), microcontroller, or any other suitabledevice for processing information or instructions. The computing device150 can be local to the user or can include components that arenon-local to the user including one or both of the processor 152 ormemory 154 (or portions thereof). For example, in at least someembodiments, the user may operate a terminal that is connected to anon-local computing device. In other embodiments, the memory 154 can benon-local to the user.

The computing device 150 can utilize any suitable processor 152including one or more hardware processors that may be local to the useror non-local to the user or other components of the computing device.The processor 152 is configured to execute instructions stored in thememory 154.

Any suitable memory 154 can be used for the computing device 150. Thememory 154 illustrates a type of computer-readable media, namelycomputer-readable storage media. Computer-readable storage media mayinclude, but is not limited to, volatile, nonvolatile, non-transitory,removable, and non-removable media implemented in any method ortechnology for storage of information, such as computer readableinstructions, data structures, program modules, or other data. Examplesof computer-readable storage media include RAM, ROM, EEPROM, flashmemory, or other memory technology, CD-ROM, digital versatile disks(“DVD”) or other optical storage, magnetic cassettes, magnetic tape,magnetic disk storage or other magnetic storage devices, or any othermedium which can be used to store the desired information and which canbe accessed by a computing device.

Communication methods provide another type of computer readable media;namely communication media. Communication media typically embodiescomputer-readable instructions, data structures, program modules, orother data in a modulated data signal such as a carrier wave, datasignal, or other transport mechanism and include any informationdelivery media. The terms “modulated data signal,” and “carrier-wavesignal” includes a signal that has one or more of its characteristicsset or changed in such a manner as to encode information, instructions,data, and the like, in the signal. By way of example, communicationmedia includes wired media such as twisted pair, coaxial cable, fiberoptics, wave guides, and other wired media and wireless media such asacoustic, RF, infrared, and other wireless media.

The display 156 can be any suitable display device, such as a monitor,screen, or the like, and can include a printer. In some embodiments, thedisplay is optional. In some embodiments, the display 156 may beintegrated into a single unit with the computing device 150, such as atablet, smart phone, or smart watch. In at least some embodiments, thedisplay is not local to the user. The input device 158 can be, forexample, a keyboard, mouse, touch screen, track ball, joystick, voicerecognition system, or any combination thereof, or the like. In at leastsome embodiments, the input device is not local to the user.

The magnetic field generator(s) 162 can be, for example, Helmholtzcoils, solenoid coils, planar coils, saddle coils, electromagnets,permanent magnets, or any other suitable arrangement for generating amagnetic field. As an example, the magnetic field generator 162 caninclude three orthogonal sets of coils to generate magnetic fields alongthree orthogonal axes. Other coil arrangement can also be used. Theoptional sensor(s) 164 can include, but are not limited to, one or moreposition sensors, orientation sensors, accelerometers, image recorders,or the like or any combination thereof.

The one or more magnetometers 160 can be any suitable magnetometerincluding, but not limited to, any suitable optically pumpedmagnetometer. Arrays of magnetometers are described in more detailherein. In at least some embodiments, at least one of the one or moremagnetometers (or all of the magnetometers) of the system is arrangedfor operation in the SERF mode.

FIG. 1B is a schematic block diagram of one embodiment of a magnetometer160 which includes a vapor cell 170 (also referred to as a “cell”) suchas an alkali metal vapor cell; a heating device 176 to heat the cell170; a light source 172; and a detector 174. In addition, coils of amagnetic field generator 162 can be positioned around the vapor cell170. The vapor cell 170 can include, for example, an alkali metal vapor(for example, rubidium in natural abundance, isotopically enrichedrubidium, potassium, or cesium, or any other suitable alkali metal suchas lithium, sodium, or francium) and, optionally, one, or both, of aquenching gas (for example, nitrogen) and a buffer gas (for example,nitrogen, helium, neon, or argon). In some embodiments, the vapor cellmay include the alkali metal atoms in a prevaporized form prior toheating to generate the vapor.

The light source 172 can include, for example, a laser to, respectively,optically pump the alkali metal atoms and probe the vapor cell. Thelight source 172 may also include optics (such as lenses, waveplates,collimators, polarizers, and objects with reflective surfaces) for beamshaping and polarization control and for directing the light from thelight source to the cell and detector. Examples of suitable lightsources include, but are not limited to, a diode laser (such as avertical-cavity surface-emitting laser (VCSEL), distributed Braggreflector laser (DBR), or distributed feedback laser (DFB)),light-emitting diode (LED), lamp, or any other suitable light source. Insome embodiments, the light source 172 may include two light sources: apump light source and a probe light source.

The detector 174 can include, for example, an optical detector tomeasure the optical properties of the transmitted probe light fieldamplitude, phase, or polarization, as quantified through opticalabsorption and dispersion curves, spectrum, or polarization or the likeor any combination thereof. Examples of suitable detectors include, butare not limited to, a photodiode, charge coupled device (CCD) array,CMOS array, camera, photodiode array, single photon avalanche diode(SPAD) array, avalanche photodiode (APD) array, or any other suitableoptical sensor array that can measure the change in transmitted light atthe optical wavelengths of interest.

FIG. 2 shows the magnetic spectrum from 1 fT to 100 μT in magnetic fieldstrength on a logarithmic scale. The magnitude of magnetic fieldsgenerated by the human brain are indicated by range 201 and themagnitude of the background ambient magnetic field, including theEarth's magnetic field, by range 202. The strength of the Earth'smagnetic field covers a range as it depends on the position on the Earthas well as the materials of the surrounding environment where themagnetic field is measured. Range 210 indicates the approximatemeasurement range of a magnetometer (e.g., an OPM) operating in the SERFmode (e.g., a SERF magnetometer) and range 211 indicates the approximatemeasurement range of a magnetometer operating in a scalar mode (e.g., ascalar magnetometer.) Typically, a SERF magnetometer is more sensitivethan a scalar magnetometer but many conventional SERF magnetometerstypically only operate up to about 0 to 200 nT while the scalarmagnetometer starts in the 10 to 100 fT range but extends above 10 to100 μT.

The systems and methods described herein will utilize OPMs, but it willbe understood that other magnetometers can be used in addition to, orinstead of, OPMs. In at least some embodiments, the OPMs utilize laserlight for pumping and probing the vapor cell of the OPM. When an arrayof OPMs is used, the laser light is delivered to each OPM vapor cell inthe array. Individual lasers could be used for the individual OPMs, butsuch an arrangement is inefficient and may result in substantial noiseas the noise in each laser can vary independently of the others. Oneconventional method for providing laser light to multiple vapor cellsutilizes fiber splitters which divide the optical beam of one or morelasers for delivery into multiple optical fibers. The optical fibersdeliver the laser light to the individual vapor cells of the OPMs. Achallenge with this approach is the large number of fibers involved, aswell as the noise generated due to polarization shifting arising frommechanical deformation of the fibers (even from polarization maintainingfibers).

Another conventional approach divides the beam from one laser into fourdifferent sensing regions within a sensor head using a diffractiveoptical element. However, this arrangement has a relatively largedistance between the diffraction element and a lens to collimate thelaser light because the deflection angle of the diffraction elementcannot be very large. In addition, the numerical aperture (NA) of thelens also limits this distance. In addition, this approach may not bescalable for systems with significantly more than four channels.

In contrast to these conventional arrangements, in at least someembodiments, a system or method includes splitting the beam(s) of one ormore surface emitting lasers using an array of beam splitters. In atleast some embodiments, the array of beam splitters forms atwo-dimensional grid.

FIGS. 3A-3C illustrate one embodiment of an array 300 of beam splitters310 (which may be in the form of cubes or any other suitable shape) foruse with an N×M array of OPMs or other magnetometers. FIG. 3B is a topview of the array 300 and FIG. 3C is a front view of one row of the beamsplitters 310. There are (N+1)×M beam splitters 310. In at least someembodiments, these beam splitters 310 are not polarizing beam splitters.In this array, one column of M beam splitters 310 a (the leftmost columnin the illustrated embodiment) are initial beam splitters that, asillustrated in FIG. 3B, divide the input beam 312 a from the lightsource 472 (see, FIG. 4) into M beams 312 b with each beam thenproceeding down a row of the array 300. In at least some embodiments,the beam splitters 310 a are arranged to produce the M beams 312 b withequal or nearly equal intensity (for example, within 1, 5, or 10% ofeach other).

The remainder (an N×M array) of the beam splitters 310 b divide the Mbeams 312 b into N×M beams 312 c (indicated by dots in FIG. 3B). In atleast some embodiments, the resulting N×M beams 312 c will have equal ornearly equal intensity (for example, within 1, 5, or 10% of each other).

The intensity of the beams 312 b, 312 c that exit each of the beamsplitters 310 can be tailored through the construction of the beamsplitter. In at least some embodiments, the use of one or more coatings(for example, adhesive, metallic, or dichroic coatings) on each beamsplitter can be used to select the amount of light transmitted throughthe beam splitter 310 and the amount of light reflected by the interface311 within the beam splitter 310. Other methods or mechanism (includingpolarization, as described below) can be used to select the amounts oftransmission and reflection of the light by the individual beamsplitters 310.

In at least some embodiments, the transmission and reflection of each ofthese beam splitters 310 a, 310 b is a function of their location in thearray and is selected so that the resulting beams are equal, orapproximately equal, in intensity. (Although a uniform beam intensity isoften useful, in other embodiments, the beams 312 b or the beams 312 cmay have different intensities and may have non-uniform beam splitting.For example, some embodiments may benefit from some vapor cellsreceiving higher intensity light than others to enhance dynamic range orthe like.) In embodiments where the intensities are equal orapproximately equal, the m-th beam splitter 310 a of the first 1×M arrayhas a reflectivity of 1/(M−m+1). Thus, the reflectivity of the firstbeam splitter 310 a is 1/M, the second beam splitter is 1/(M−1), thethird beam splitter is 1/(M−2), and so forth. The transmission for them-th beam splitter 310 a is 1−1/(M−m+1). This arrangement leads to beams312 b each having a relatively equal value of reflected light from thearray with each beam having ideally 1/M of the input laser power.

Each of the beams 312 b go through a similar reflection and transmissionprocess along one of the rows of the array of N×M beam splitters 310 b.In each row, the n-th beam splitter 310 b (excluding the first beamsplitter from this count) of the row has a reflectivity of 1/(N−n+1).The transmission for the n-th beam splitter 310 b is 1−1/(N−n+1). Thisarrangement produces an array of N×M beams 312 c with almost equalpower, each approximately 1/NM of the original laser power. (Forinstances in which N=M, the result is N² beams with each beam havingapproximately 1/N² of the original laser power.) The orientation of thebeam splitters 310 b in the N×M array is such that each of these beams312 c will propagate perpendicular to the plane of the array 300, asshown in FIG. 3C.

FIG. 4 illustrates a portion of magnetic field measurement system with alight source 472 (such as a laser, for example, an edge emitting laser),a lens 461 (for example, a collimating lens), a polarizer 463, the array300 of beam splitters 310, a quarter waveplate 465, an array of vaporcells 470, and an array of detectors 474. Each of the beams 312 c (FIG.3C) is directed from the array 300 of beam splitters 310 through thequarter waveplate 465 into the array of N×M vapor cells 470 enablingsimultaneous operation of all of the OPMs. An N×M array of detectors 474receives the light that passes through the vapor cells 470. Theembodiment of FIG. 4 is just one example of an arrangement, it will beunderstood that other arrangements can be used.

In at least some embodiments, the light beam 312 a of the light source472 (for example, a surface emitting laser) can be coupled through afiber (not shown) to the collimating lens 461. This arrangement may beparticularly useful if the light source cannot be positioned close tothe magnetometer array (for example, due to the production of a magneticfield by the light source).

To improve the performance of the magnetometers (e.g., OPMs), areference detector 580 can be added within the beam path length, asillustrated in FIG. 5. A small portion (for example, no more than 0.5,1, 2, 5, or 10%) of the initial light beam 312 a, which has passedthrough a polarizer 561, is allowed to pass through all of the initialbeam splitters 310 a to be detected by the reference detector 580. Thelight detected by the reference detector 580 can be used, for example,to monitor variations in the light beams 312 a, 312 b, 312 c arisingfrom the light source.

FIG. 6 illustrates another embodiment of an array 300 of polarizing beamsplitters 310 with waveplates 682, 684 positioned between the polarizingbeam splitters 310 and an initial polarizer 661. A polarizing beamsplitter 310 may transmit one polarization of light and reflect anotherpolarization of light (for example, s- and p-polarizations).

This embodiment utilizes polarization as the basis for splitting thelight beam 312 a into M light beams 312 b and then into N×M light beam312 c. The waveplates 682, 684 are selected to rotate the polarizationof the light beam prior to each of the polarizing beam splitters 310 toproduce the desired amount of light reflection by the polarizing beamsplitter by altering the amount of light in the first and secondpolarizations. In at least some embodiments, the amount of rotation canbe selected using the thickness of the waveplate 682, 684 or thematerials of the waveplate or any combination thereof.

In at least some embodiments, the waveplates 682, 684 are selected toresult N×M light beams 312 c with equal or approximately equal intensityusing the formulas provided above for the reflection of light at eachbeam splitter. The reflection formulas presented above lead directly tothe amount of polarization rotation to be accomplished by each of thewaveplates 682, 684. In embodiments where the intensities are equal orapproximately equal, the waveplate 682 prior to the m-th polarizing beamsplitter 310 a of the first 1×M array rotates the polarization of thelight beam so that the fraction of the light reflected by the m-thpolarizing beam splitter is 1/(M−m+1). In each row, the waveplate 684prior to the n-th polarizing beam splitter 310 b (excluding the firstpolarizing beam splitter from this count) of the row rotates thepolarization of the light beam so that the fraction of the lightreflected by the n-th polarizing beam splitter is 1/(N−n+1). Thetransmission for the n-th beam splitter 310 b is 1-1/(N−n+1).

In at least some embodiments, the beam splitters 310 of the array 300cubes may be fused together using an optical adhesive such as, forexample, an index-matched optical glue. (In the embodiment of FIG. 6,the beam splitters 310 and waveplates 682, 684 can be fused togetherusing an optical adhesive.) This arrangement may increase the mechanicalstability of the array 300 resulting in a single solid unit. Thearrangement may also reduce loss and interference effects due toreflection from the surfaces or facets of each beam splitter.

In at least some embodiments, an array of beam splitters can beconstructed using specifically cut material for a smaller number ofparts. This arrangement can reduce the number of parts for the N×Noutput channels, from 12 parts to only N parts. One embodiment ispresented in FIGS. 7A to 7D. The first step is to cut glass parts intoprisms 790 as illustrated in FIG. 7A. In at least some embodiments, thewidth 791 of the prism 790 is a, the length 792 of the prism is a×N, theheight 793 of the prism is √{square root over (2)}a, and the angle 794is 45 degrees. The glass prisms 790 are coated on their largest areasurfaces with optical material coatings that will produce the correctreflectivity and transmission for both s-polarization andp-polarization, as illustrated in FIG. 7B. The yield of this step can beimproved by stacking the glass prisms as illustrated in FIG. 7B with anumber of prisms coated at once. As illustrated in FIG. 7C, differentcoatings can be used to produce the different prisms 790, 790 a, 790 b,790 c. The prisms 790, 790 a, 790 b, 790 c are then stacked in thedesired order such as, for example, with the prism's reflectivityordered as described in the reflectivity formulas above. These prisms790, 790 a, 790 b, 790 c can be bonded permanently using any suitablemethod, such as optical bonders or adhesive 795, to make a single pieceas illustrated in FIG. 7D.

In at least some embodiments, the systems and methods described hereincan produce a relatively large number (for example, 12, 16, 25, 36, 64,100 or more) of output beams from a single light source. In at leastsome embodiments, multiple light sources can be used with an array ofbeam splitters associated with each light source. In at least someembodiments, these systems and methods can have a higher tolerance tomechanical vibration, thermal fluctuation, or polarization fluctuationas compared to the conventional systems described above. The systems andmethods may also present an arrangement with a more compact volume for agiven number of output channels.

In at least some embodiments, when compared to an N×N array of surfaceemitting lasers, the systems and methods can utilize and arrangementthat can produce larger power per output channel for up to about N² (forexample, approximately 100) channels. Moreover, controlling the powerstability and wavelength stability of N² lasers is much more complicatedthan a single laser, as the array size (N²) grows. Finally, the currentto power each of the N² lasers, and their thermal controller circuits,can produce complex magnetic fields near each of the OPM cells. Incontrast, the systems and methods described herein replace thesemagnetic field producing components with zero field (and lowpermeability) glass. Therefore, the magnetic field inhomogeneity nearthe OPM array for the systems described herein may be significantlylower than would be the case for an N×N array of lasers.

In at least some embodiments, the systems and methods described hereincan have one or more of the following features: compact design,relatively high mechanical stability, relatively low mechanicallyinduced noise, and relatively high polarization stability.

In at least some embodiments, components of system can be part of awearable article, such as a helmet, hood, cap, or other shapeconformable to a user's head. For example, the vapor cells, array ofbeam splitters, and detectors can be part of the wearable and portablearticle. In some embodiments, the light source may also be part of thewearable and portable article.

Examples of magnetic field measurement systems in which the embodimentspresented above can be incorporated, and which present features that canbe incorporated in the embodiments presented herein, are described inU.S. Patent Application Publications Nos. 2020/0072916; 2020/0056263;2020/0025844; 2020/0057116; 2019/0391213; 2020/0088811; 2020/0057115;2020/0109481; 2020/0123416; and 2020/0191883; U.S. patent applicationSer. Nos. 16/741,593; 16/752,393; 16/820,131; 16/850,380; 16/850,444;16/884,672; 16/904,281; 16/922,898; and Ser. No. 16/928,810, and U.S.Provisional Patent Application Ser. Nos. 62/689,696; 62/699,596;62/719,471; 62/719,475; 62/719,928; 62/723,933; 62/732,327; 62/732,791;62/741,777; 62/743,343; 62/747,924; 62/745,144; 62/752,067; 62/776,895;62/781,418; 62/796,958; 62/798,209; 62/798,330; 62/804,539; 62/826,045;62/827,390; 62/836,421; 62/837,574; 62/837,587; 62/842,818; 62/855,820;62/858,636; 62/860,001; 62/865,049; 62/873,694; 62/874,887; 62/883,399;62/883,406; 62/888,858; 62/895,197; 62/896,929; 62/898,461; 62/910,248;62/913,000; 62/926,032; 62/926,043; 62/933,085; 62/960,548; 62/971,132;62/983,406; 63/031,469; and 63/037,407, all of which are incorporatedherein by reference.

Further details discussing different form factors in small, portable,wearable devices and applications thereof are set forth in U.S. patentapplication Ser. Nos. 16/523,861 and 16/364,338, and U.S. ProvisionalPatent Application Ser. Nos. 62/829,124; 62/839,405; 62/894,578;62/859,880; and 62/891,128, all of which are incorporated herein byreference, as well as other references cited above.

The above specification provides a description of the invention and itsmanufacture and use. Since many embodiments of the invention can be madewithout departing from the spirit and scope of the invention, theinvention also resides in the claims hereinafter appended.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. An array of optically pumped magnetometers,comprising: an array of vapor cells; an array of beam splitters, whereinthe array of beam splitters is arranged into a plurality of columns,including a first column, and a plurality of rows, wherein each row andeach column comprises at least two of the beam splitters, wherein thearray of beam splitters is configured to receive light into the firstcolumn of the array and to distribute that light from the first columninto each of the plurality of rows and to distribute the light from eachof the rows into a plurality of individual light beams directed towardthe vapor cells, wherein the first column has M of the beam splittersand the m-th one of the beam splitters in the first column has areflectivity of 1/(M−m+1), wherein M is an integer greater than one andm is an integer ranging from 1 to M; and a quarter waveplate disposedbetween the array of beam splitters and the array of vapor cells.
 2. Thearray of optically pumped magnetometers of claim 1, wherein the array ofvapor cells is an N×M array and the array of beam splitters is an(N+1)×M array, wherein N and M are integers greater than one.
 3. Thearray of optically pumped magnetometers of claim 2, wherein the array ofbeam splitters is configured to generate N×M beams of light.
 4. An arrayof optically pumped magnetometers, comprising: an array of vapor cells;an array of beam splitters, wherein the array of beam splitters isarranged into a plurality of columns, including a first column, and aplurality of rows, wherein each row and each column comprises at leasttwo of the beam splitters, wherein the array of beam splitters isconfigured to receive light into the first column of the array and todistribute that light from the first column into each of the pluralityof rows and to distribute the light from each of the rows into aplurality of individual light beams directed toward the vapor cells,wherein the array of vapor cells is an N×M array and the array of beamsplitters is an (N+1)×M array, wherein N and M are integers greater thanone, wherein the array of beam splitters is configured to generate N×Mbeams of light, wherein the N×M beams of light have intensities thatdiffer by no more than 5% from each other.
 5. The array of opticallypumped magnetometers of claim 4, wherein the first column has M of thebeam splitters and the m-th one of the beam splitters in the firstcolumn has a reflectivity of 1/(M−m+1), wherein M is an integer greaterthan one and m is an integer ranging from 1 to M.
 6. The array ofoptically pumped magnetometers of claim 1, wherein at least one row hasa one of the beam splitters from the first column followed by N of thebeam splitters after, wherein the n-th one of the N beam splitters has areflectivity of 1/(N−n+1), wherein N is an integer greater than one andn is an integer ranging from 1 to N.
 7. The array of optically pumpedmagnetometers of claim 1, wherein the beam splitters are polarizing beamsplitters.
 8. A magnetic field measurement system, comprising the arrayof optically pumped magnetometers of claim 1; and a light sourceconfigured and arranged to direct light into the first column of thearray of beam splitters.
 9. The magnetic field measurement system ofclaim 8, further comprising a polarizer disposed between the lightsource and the array of beam splitters.
 10. The magnetic fieldmeasurement system of claim 8, further comprising an array of lightdetectors configured to receive light passing through the vapor cells.11. An array of optically pumped magnetometers, comprising: an array ofvapor cells; an array of beam splitters, wherein the array of beamsplitters is arranged into a plurality of columns, including a firstcolumn, and a plurality of rows, wherein each row and each columncomprises at least two of the beam splitters, wherein the array of beamsplitters is configured to receive light into the first column of thearray and to distribute that light from the first column into each ofthe plurality of rows and to distribute the light from each of the rowsinto a plurality of individual light beams directed toward the vaporcells, wherein the array of vapor cells is an N×M array and the array ofbeam splitters is an (N+1)×M array, wherein N and M are integers greaterthan one, wherein the array of beam splitters is configured to generateN×M beams of light, wherein the N×M beams of light have intensities thatdiffer by no more than 5% from each other; and a reference detectorconfigured to receive light that has passed through the beam splittersof the first column.
 12. The array of optically pumped magnetometers ofclaim 11, wherein the first column has M of the beam splitters and them-th one of the beam splitters in the first column has a reflectivity of1/(M−m+1), wherein M is an integer greater than one and m is an integerranging from 1 to M.
 13. The array of optically pumped magnetometers ofclaim 11, wherein at least one row has a one of the beam splitters fromthe first column followed by N of the beam splitters after, wherein then-th one of the N beam splitters has a reflectivity of 1/(N−n+1),wherein N is an integer greater than one and n is an integer rangingfrom 1 to N.
 14. The array of optically pumped magnetometers of claim11, wherein the beam splitters are polarizing beam splitters.
 15. Amagnetic field measurement system, comprising the array of opticallypumped magnetometers of claim 11; and a light source configured andarranged to direct light into the first column of the array of beamsplitters.
 16. The magnetic field measurement system of claim 15,further comprising a polarizer disposed between the light source and thearray of beam splitters.
 17. The magnetic field measurement system ofclaim 15, further comprising an array of light detectors configured toreceive light passing through the vapor cells.
 18. The array ofoptically pumped magnetometers of claim 4, wherein at least one row hasa one of the beam splitters from the first column followed by N of thebeam splitters after, wherein the n-th one of the N beam splitters has areflectivity of 1/(N−n+1), wherein N is an integer greater than one andn is an integer ranging from 1 to N.
 19. The array of optically pumpedmagnetometers of claim 4, wherein the beam splitters are polarizing beamsplitters.
 20. A magnetic field measurement system, comprising the arrayof optically pumped magnetometers of claim 4; and a light sourceconfigured and arranged to direct light into the first column of thearray of beam splitters.